My fear of losing my job once I became a mother was not, in any way, assuaged by the fact that my replacement during maternity leave was a friend. The magazine editorship I'd fought hard for (More! Magazine, way back in 1999) was certainly going to be in safe hands - but would it still be mine a few months later when I came back to work?

I went off on maternity leave a mess of insecurities. The interim editor was a brilliant journalist, and at the time unencumbered with inconvenient children - as I was about to be. What if she turned out to be better than me, or more popular? What if the circulation figures went up - what if they gave her my job?

I'd heard the stories of women who'd been sidelined or found their role dramatically reduced after starting a family. One friend returned to her job in market research after six months at home with her baby son to discover an intern had taken over the exciting parts of her role. He could stay late every night whereas she had to charge off at 5pm to collect her baby from the child minder, so while the job title was still hers she had, to all intents and purposes, been replaced. It's not meant to happen, and it's certainly not legal, but happen it does.

A National Childbirth Trust survey found that one in three women returning to work after maternity leave find their job 'unrecognisable' to the one they left. According to research by law firm Slater & Gordon, nearly seven in ten working mums claim to have been taken off their career path and put on the 'Mummy Track', where they were side-lined as less experienced colleagues were often promoted ahead of them.

This is probably why BBC 1's The Replacement strikes such a chord with women like me. In the three-part drama, pregnant architect Ellen views her charming and enthusiastic interim replacement Paula as a threat. In spite of being a super-safe pair of hands to entrust with her beloved job, Paula makes Ellen seethingly paranoid - probably because she organises meetings to which Ellen is not invited, and makes changes to her architectural plans without involving her - all in the name of 'taking the pressure off her' during her pregnancy.

"It's classic passive aggressive behaviour," says assertiveness expert Antoinette Dale Henderson author of Leading With Gravitas: Unlock The Six Keys To Impact And Influence. "Passive aggression is like an iron fist in a velvet glove. You receive a punch, but it feels a bit like you're also being stroked".

The Replacement is an extreme iteration of the issue, of course - it's TV. Paula turns out to be a deeply disturbed individual, but Ellen's rising fears remind me that all of us are terrified of being replaced. Who hasn't felt threatened by a colleague or junior staffer who's so 'keen' to help out that they seem to be muscling in on your territory, so that before you know it they're getting the credit for your work or taking over some of your duties? And all with a big smile on their face so you find it hard to call them out?

Whether pregnant or not, it's helpful to know how to face down a passive aggressive co-worker who seems to have his/her eye on the prize that is your job, or who's manipulating the situation to ensure they look good at the expense of your reputation.

According to Antoinette, what often drives passive aggressive behaviour is a desire for attention that's unfulfilled. It's a vicious coping mechanism for insecurity.

"To avoid this situation in the first place, it's important to clarify roles and responsibilities so there are minimal grey areas around who is responsible for what. If it's obvious that a colleague is taking over your role or muscling in on your responsibilities, collate examples of what they're doing and then have a one to one meeting to highlight your concerns. As well as telling them to 'back off', it's important to make sure they know what you want them to do instead," says Antoinette. "Ideally identify a challenging project that will give them kudos, so they can channel their energies into that".

I was one of the lucky ones. A few months after becoming a mum I returned to More! to find my position relatively unchanged - but I can't say the same for me. Suddenly acutely aware that the world of naked male centrefolds and Position of the Fortnight was a bad mix with Pampers and Calpol, I didn't last too much longer. But at least when I left it was of my own accord.